An optical fiber with lens is an optical fiber coupling component which performs high-efficiency optical coupling of light incident on an optical fiber or light emitted from the optical fiber, and is used for optical coupling between optical fibers, optical coupling between an optical fiber and a light emitting/light receiving element, optical coupling between an optical fiber and an optical component (such as an optical waveguide or an optical integrated circuit), and the like.
A GRIN lens fused and connected to an optical fiber is known as a conventional optical fiber with lens. GRIN lens is short for gradient index lens which is a columnar (rod-like) lens with a refractive index distribution n(r) in a direction of a radius r satisfying n(r)=n0(1−(½)(g·r)2). In this case, n0 denotes a central refractive index and g denotes a constant (a refractive index distribution constant) that represents a light condensing ability of the GRIN lens. When a radius of the GRIN lens is denoted by R, a minimum refractive index (minimum distribution refractive index) nt of the refractive index distribution is a refractive index in an outer edge part of the lens, satisfying nt=n(R)=n0(1−(½)(G·R)2).
Since a GRIN lens has a columnar shape, an optical fiber with lens using a GRIN lens provides an advantage in that mechanical axis alignment during optical coupling is facilitated by making an outer diameter of the optical fiber and an outer diameter of the GRIN lens approximately the same, and also provides an advantage of improving space efficiency when arranging a plurality of optical fiber with lens in parallel to form an array.
An optical fiber with lens using a GRIN lens is generally used as a collimator which collimates light from an optical fiber or a condenser which condenses light from an optical fiber. In particular, examples of use as a condenser include optical coupling between a light emitting element (for example, a semiconductor laser) and an optical fiber. Such a case requires a GRIN lens of which light condensing ability is high enough to sufficiently cover an emission angle of the light emitting element and also requires that light incident on the GRIN lens is incident on a core of the optical fiber within a critical angle of the optical fiber.
As an optical fiber with lens which accommodates such requests, an optical fiber with a GRIN lens is developed in which a GRIN lens with a relatively low NA is connected to an end face of an optical fiber and a GRIN lens with a relatively high NA is connected to an end face of the GRIN lens with the relatively low NA (refer to PTL 1 below). In this case, NA is short for numerical aperture and, using g described earlier and a radius R and a central refractive index n0 of a GRIN lens, NA is expressed as NA=n0·g·R.